Previous studies in Dr. Hill's lab have found that during a low activity day, fat oxidation varies directly with the size of the fat mass, so that subjects with a high activity level show a lower rate of fat oxidation on a sedentary day than do sedentary subjects. If this is the case, then for fat oxidation to be higher in regular exercisers than in sedentary subjects, much of the fat oxidation in exercisers must occur during exercise and during non-exercise times when subjects are studied on a typical activity day. Our hypothesis is that the proportion of fat oxidation occurring during exercise will increase with increasing level of habitual physical activity. Active subjects will also be studied on a low activity day in order to test the hypothesis that non-exercise related fat oxidation is directly proportional to the size of the fat mass. Rates of energy expenditure and macronutrient oxidation only consider one side of the energy balance equation. A complete understanding of how physical activity contributes to the maintenance of body weight and fat mass will only be gained by considering both energy intake and energy expenditure. Physical activity increases total daily energy expenditure, but most physically active individuals are weight stable, suggesting that energy intake must also be high. Cessation of exercise would thus predispose physically active individuals to rapid weight and fat mass gain unless there were a rapid adjustment of energy intake to the reduced physical activity. For example, when active subjects experience a sedentary day, they would be in positive energy and fat balance unless they spontaneously ate less on this day. Thus, we intend to determine if energy intake is adjusted to a decline in energy expenditure produced by cessation of exercise. If not, sedentary days would pose a particular risk for regular exercisers in terms of risk of showing positive fat balance.